Thoughts on some HBO HD broadcasts : 810i ?

I was watching HBO HD the other day, and Flight of the Conchords came on. I noticed that although this was a 1080i broadcast, and the aspect ratio of the show appeared to be a regular 16:9, the whole show was windowboxed top to bottom, left to right:I know this is Entourage (which doesn't display like this), but you get the idea. The gray colors are for emphasis, you most likely see black all the way around. You might have noticed the same thing about HBO HD commercials too. My Sony TV has a "zoom" feature that zooms in on a 16:9 shaped box right in the middle of the screen, so that the letterboxing is perfectly offset and I have fullscreen. This got me to thinking: what effective resolution am I getting out of this ?

Some Quick Calculations
The whole frame is 1920x1080. Its pretty clear that the lighter gray box is a 4:3 box maximally fitting the larger box. The height is 1080, and if it is in a 4:3 proportion, then the width of the smaller box is 1080/3 * 4 = 1440. So we're down to 1440x1080. The content itself is 16:9, maximally fitting in THIS box. So if the width of the content is 1440, the height is 1440/16 * 9 = 810. So we're finally down to 1440x810.

Keeping in mind that the original broadcast was interlaced, we could easily call this resolution 810i (even though this isn't a broadcast standard). So when I use my TV's zoom/scaling feature, what I'm seeing is broadcast 810i. Pretty interesting...companies like HBO probably use this little shortcut to get away with putting out a broadcast comparable in quality to 720p, while saving on bandwidth over a 1080i channel.

Conclusion
Who knows, its just a random thought I had. Feel free to comment with your thoughts...